The invention relates generally to a method and arrangement for agitating baths and for controlling the temperature of the same. Of particular interest to the invention is the regulation of melts, such as molten salts, which are used for the heat treatment of metallic articles.
One purpose for which melts, such as molten salts, are utilized is to obtain a controlled colling of hot rolled steel wire so as to permit the latter to undergo a microstructural transformation.
It has already been proposed to introduce rolled wire into a salt melt having a temperature of 450.degree. to 600.degree. C after the wire has left the last stage of a rolling mill. If necessary, the wire may be subjected to a precooling operation subsequent to leaving the rolling mill and before the introduction thereof into the melt. The purpose of the melt is to rapidly cool the wire to the appropriate temperature range for the desired microstructural transformation and to maintain the wire at a constant temperature until the end of the transformation. Wire which has been treated in this manner possesses a tempered martensitic (sorbitic) structure which is well-suited for cold deformation of the wire by drawing.
Due to the existence of a large number of unsolved technological problems, the heat treating procedure outlined above has not heretofore been able to find application in large industrial production installations. One of these problems is associated with the large quantities of heat which are brought into the melt by the hot wire which is to be heat treated. Thus, it has not been possible to remove this heat from the melt in such a manner that, within the cycle of operation of the rolling mill, the temperature of the melt is maintained so constant within narrow limits that an optimum and reproducible microstructure is obtained. It is necessary, in order to obtain an optimum and reproducible microstructure, that the heating, and especially the cooling, of the melt be continuously and rapidly changed and, in particular, that the heating and the cooling of the melt be changed in dependence upon the operating cycle of the rolling mill.
A further problem results from the fact that it is necessary to cause such motion and mixing of the melt that accumulations of heat occur neither in the vicinity of the location where the wire is introduced into the melt nor in the regions of the coils which are formed from the wire prior to the introduction thereof into the melt. Thus, on the one hand, localized temperature increases may cause an undesired thermal decomposition of the reactive melt, and reactions with the container accommodating the melt, as well as with the material to be treated, may occur. On the other hand, localized temperature increases cause variable cooling conditions which, in turn, may adversely affect the quality of the wire by causing a nonhomogeneous microstructure to be obtained.
Conventionally, an external circulatory system is utilized for removing heat from the melt. Such a system generally includes fluid-conveying pumps and heat-exchangers. In practice, however, an external system of this type is incapable of removing the requisite quantities of heat from the melt, is incapable of generating the requisite agitation of the melt and, in particular, is incapable of rapid adjustment to the cooling and heating conditions required for the melt. Moreover, technical operating difficulties arise when using an external circulatory system due to the formation of incrustations and blockages. Aside from the high maintenance costs involved with arrangements of this type, they are exceedingly expensive to regulate and are of extremely costly construction.
Another unsolved problem relating to the heat treating procedure outlined above results from the fact that considerable quantities of the melt are dragged along with the wire when the latter is removed from the melt. Crusts are thus formed on the wire and these crusts usually consist of salt and salt mixtures. It is necessary, before further processing of the wire, to remove these crusts from the wire by means of a water rinse. However, the water used for the rinsing operation has a high concentration of salt and cannot, therefore, be simply discharged as waste water due to environmental considerations. Nor is it economically feasible to treat the rinsing water in order to recover the salt therefrom. It is for this reason also that a procedure for the heat treatment of wire by means of a salt melt directly after hot rolling has not been able to find application heretofore.